Semiconductor devices are built up using a number of material layers. Each layer is patterned to add or remove selected portions to form circuit features that will eventually make up an integrated circuit. Some layers can be grown from another layer; for example, an insulating layer of silicon dioxide can be grown over a layer of silicon by oxidizing the silicon surface. Other layers are formed using deposition techniques, typical ones being chemical vapor deposition (CVD), evaporation, and sputtering.
Deposition methods form layers using vaporized materials that condense to form a film on the surface of interest. Unfortunately, the films thus formed are not limited to the surface of interest, but tend also to form on other surfaces within the reaction chamber. Thus, after substantial use, a thick film of the deposited material accumulates on components and surfaces within the reaction chamber. These films eventually become troublesome sources of contaminants. Etch processes also contaminate inside surfaces of reaction chambers, though by different mechanisms. In either case, the reaction chamber, including internal components, must be periodically cleaned or replaced.
Many process contaminants are removed using hazardous liquids. Unfortunately, the storage, use, and disposal of hazardous liquids and their vapors are dangerous and expensive, particularly when these chemicals are used in large volumes. There is therefore a need for cleaning methods and systems that minimize the required amounts of hazardous chemicals.
The difficulty and expense of dealing with hazardous chemicals are not the only problems encountered when cleaning semiconductor process equipment. Some forms of contamination are so stubbornly attached to the underlying material that removal of the contamination jeopardizes the part to be cleaned. Each of FIGS. 1, 2, and 3 (prior art) illustrates an exemplary component and is used to describe a particular cleaning problem addressed in the following disclosure.
FIG. 1 (prior art) depicts a stainless-steel shield 100 used to contain titanium-bearing vapors during physical vapor deposition (PVD) processes used to deposit layers of titanium and titanium alloys on semiconductor wafers. In confining such vapors, the interior surface 105 of shield 100 becomes highly contaminated with layers of titanium and titanium species, such as titanium nitride. Exterior surface 110 of shield 100 also becomes contaminated, though to a lesser extent. Shield 100 must therefore be periodically cleaned or replaced.
Conventional etchants that attack the titanium and titanium alloys also attack stainless steel. Immersing shield 100 in these etchants to remove the contaminants can therefore damage the underlying stainless steel. Exterior surface 110 is particularly vulnerable because that stainless steel lacks the thick contaminant layer of interior surface 105, and is thus exposed to etchants for a longer time. Pitting and roughening of exterior surface 110 is undesirable for aesthetic purposes and because rough surfaces trap undesirable contaminants when shield 100 is returned to a process chamber. There is therefore a need for a method of effectively removing titanium contaminant species from shield 100 without damaging the underlying stainless steel.
FIG. 2 (prior art) depicts an aluminum blocker plate 200 used to distribute gases evenly over a semiconductor surface. Blocker plate 200 is used, for example, to evenly distribute silicon-bearing gases (e.g. silane) over the surface of a semiconductor wafer during silicon deposition processes. Blocker plate 200 includes a constellation of small holes 205 through which pass the silicon-bearing gas. During such deposition processes, the surfaces of aluminum blocker plate 200, including the inner surfaces of holes 205, become contaminated with silicon and silicon oxides. Blocker plate 200 must therefore be periodically cleaned or replaced.
Oxides of silicon are difficult to remove from aluminum because common silicon etchants vigorously attack aluminum. A similar problem exists for components of or layered with yttrium oxide or sprayed ceramic. Expensive components like blocker plate 200 are therefore discarded and replaced rather than cleaned and reused. There is therefore a need in the art for a way to remove silicon and silicon-bearing contamination from expensive aluminum, yttrium oxide, and sprayed ceramic parts.
FIG. 3 (prior art) depicts a diffusion tube employed in high-temperature furnaces to deposit polysilicon and silicon nitride on semiconductor wafers. Diffusion tube 300 can be of quartz or silicon carbide. During the deposition of polysilicon or silicon nitride, these deposited materials built up on the inner surfaces of diffusion tube 300. After a period of use, the resulting contamination layers can begin to flake off, posing a serious threat of induced defects on the wafers being processed. It is therefore necessary to periodically clean or replace diffusion tube 300.
Unfortunately, current methods of cleaning diffusion tubes are inadequate. In a typical process, one or more “spray balls” are inserted up into a vertically positioned diffusion tube 300. Etchants are then sprayed against the interior surfaces of diffusion tube 300 to dissolve away the accumulated contamination layers. Spray balls do not apply chemicals evenly; therefore contamination removal is slow and uneven. There is therefore a need of improved methods of restoring expensive diffusion tubes to a contamination-free state.
The examples of FIGS. 1-3 are in no way exhaustive of the problems encountered as a result of contaminated semiconductor-processing equipment or of the types of parts that can be cleaned. Many other expensive components pose difficult cleaning problems. For example, some titanium components become contaminated with titanium species, including titanium metal and titanium nitride. Known methods of removing titanium species are labor intensive and potentially damage the underlying titanium substrate. There is therefore a need for methods of removing titanium metal and titanium alloys from titanium substrates.